Arsenic is a major contaminant of soils, sediments, wastes, and water in the United States and in foreign countries. Contamination of soils results from, for example, pesticides and wood treatments. Not only is arsenic a prevalent contaminant but it is also particularly dangerous because it is a known carcinogen. Currently there is no cost effective and efficient way to clean up soils contaminated with arsenic.
The use of arsenic in agricultural and industrial processes has resulted in numerous contaminated soils in Florida. During the early part of the 20.sup.th century, arsenic was commonly used as an insecticide component to control disease-carrying ticks on southern cattle so that Florida cattlemen could sell to the northern cattle markets. Arsenic, typically in the form of arsenic pentoxide, was also used in conjunction with copper sulfate and sodium or potassium dichromate as a wood preservative which is known as the copper/chromium/arsenic wood preservative process, or CCA (Grant and Dobbs 1977). With both of these processes, the risk of soil contamination from spills and leaks was great. The arsenic level at many of these sites is currently higher than 600 mg kg.sup.-1 even after years of idleness. The typical concentration range in soil is between 0.1 to 40 mg kg.sup.-1 with a mean concentration of 5-6 mg kg.sup.-1 (Kabata-Pendias and Pendias, 1992). The typical range of arsenic in Florida soils is 0.01 to 6.1 mg kg.sup.-1 (Ma et al. 1997).
Thus, environmental arsenic contamination is of concern due to its biological activities as a teratogen, carcinogen, and mutagen as well as its detrimental effects on the immune system (Squibb and Fowler 1983). Efforts to remediate these arsenic contaminated soils have been minimal, primarily due to the lack of technologies and the costs associated with the excavation and landfilling of the soil materials.
In most soil systems, arsenic is present in many forms of which arsenate is typically the dominant one. In this form, it has properties very similar to phosphate including the formation of insoluble salts with cations and sorption by soil constituents. Because arsenic has a wide range of oxidation states (-3,0,+3, and +5) it has the ability to form many types of organic and inorganic complexes. At high pH ranges, typically 7 to 9, the arsenic in soils predominantly consists of complex oxyanions of As(V), such as AsO.sub.2 --, AsO.sub.4.sup.-3, HAsO.sub.4.sup.-2, and H.sub.2 AsO.sub.4.sup.-1. In soils with low pH and low Eh, the predominant forms of arsenic are the arsenites (H.sub.3 AsO.sub.3) (Kabata-Pendias and Pendias, 1992).
Although arsenic is commonly found in all natural systems at minute levels, it can be very toxic to both plants and animals at higher concentrations. The toxic effects of arsenic have been known for some time. The exposure of animals to arsenic is second in toxicity only to lead for many farm and household animals. Most cases of arsenic poisoning in animals occur in bovine and feline species as a result of contaminated feed supplies. Other species that are affected are forage-eating animals, such as horses and sheep, that encounter fields that may have been treated with arsenic pesticides. The toxic effects of arsenic to humans and animals can be related to the interactions that occur within the cells of poisoned individuals, especially the mitochondria.
Arsenic is present naturally in almost all plant and tree species in minute amounts. The tolerance of plant and tree species to arsenic varies with species, soil type, and the form of arsenic present in a soil (Porter and Peterson, 1977). Over time, a classification scheme was developed to identify the tolerance of vegetables and fruit species (Table 1). In general, the distribution of arsenic in the plant species follows a common trend. Typically, the roots will contain higher concentrations of arsenic than the stems, leaves, and fruits. Some plant species have demonstrated the ability to accumulate elevated arsenic in the above ground portion of the plants. Porter and Peterson (1977) identified that some species in the Agrostis genus had the ability to accumulate up to 3,460 mg kg.sup.-1 As from soil that contained up to 2.6% arsenic. Other reports have demonstrated the ability of Douglas fir, Pseudotsuga menziesii, to accumulate up to 10,000 mg kg.sup.-1 As in ash, allowing this tree to be used as a biogeochemical indicator for gold, silver, and other ores (Fowler, 1977; Cullen and Reimer, 1989).
TABLE 1 Arsenic Tolerance of Agronomic Crops Tolerance Grouping Crop Species Very Tolerant Asparagus, potato, tomato, carrot, tobacco, dewberry, grape, red raspberry Fairly Tolerant Strawberry, sweet corn, beet, squash Low or No Tolerance Snap pea, lima bean, onion, pea, cucumber, alfalfa Source: Walsh and Keeney (1975)
Due to the concern expressed over arsenic contaminated sites, various remediation techniques have been developed. Methods for remediating arsenic contaminated soil can be performed in situ and ex situ and have varying degrees of complexity, effectiveness, and cost. These methods can be divided into three techniques: chemical, physical, and biological remediation methods.
One of the biological remediation techniques is phytoremediation. Phytoremediation is a growing technology that utilizes the ability of plants to accumulate nutrients and trace elements. Phytoremediation is the process of employing plants to remediate contaminated soils. Typically this is done in one of two ways, either by phytostabilization or by phytoextraction (Bolton and Gorby 1995). With phytostabilization, plants are used to stabilize contaminated soils by decreasing wind and water erosion as well as decreasing water infiltration and contaminant leaching into groundwater. Phytoextraction attempts to remove contaminants from the rhizosphere through plant uptake and the contaminants are accumulated in roots, leaves and/or stems. The plant materials are then harvested and the contaminants reclaimed from the plant biomass or the materials are disposed of at a hazardous waste facility.
Currently, certain plants have been identified that can be utilized to remediate soil and water systems contaminated with metals, metalloids, petroleum constituents, pesticides, and industrial wastes (Dix et al., 1997; Ebbs et al., 1997; Lasat et al., 1998). Also, many plant species have been identified that accumulate lead, selenium, nickel, zinc, and other metals. For example, U.S. Pat. Nos. 5,364,451 and 5,711,784 describe phytoremediation of metal-containing soils. McGrath et al. (1997) demonstrated the effective removal of cadmium and zinc by plant species Thlaspi caerulescens. Kramer et al. (1997) found that Thlaspi goesingense (Halacsy) removes nickel from contaminated soils.
For remediation of contaminant sites and/or recovery of precious metals, phytoextraction can be an attractive option. Phytoextraction is the process of removing a contaminant from a system via plant roots for remediational purposes. Originally, the term phytoextraction was applied to the removal of trace elements from soils, but recently new applications have been discovered for this process. One of the newest uses of phytoextraction has been its use in accumulating trace elements of economic value, such as gold and nickel.
In some situations, soil amendments and chelating agents can be used to aid in plant growth and in accumulation of trace elements by plants. The soil may have a low pH, poor aeration, inappropriate soil texture, high salinity, etc. To overcome this, agronomic techniques can be used to increase the chance of plant survival. These include addition of organic matter, liming, and fertilization to name a few. In certain situations, addition of soil amendments decreases the quantity of the contaminant that the plant will accumulate but this is typically offset by the increase in biomass that is produced (Bennett, 1998).
For many soil contaminants, chelating agents or organic acids are required to assist in their accumulation by plants. The low solubility of many trace elements and radionuclides is often the limiting factor in metal extraction by plants (Huang et al., 1998). For example, lead in soil has a limited solubility and low bioavailability for plant uptake due to complexation with organic matter, sorption on clay and oxides, and precipitation as carbonates, hydroxides, and phosphates.
To overcome this problem, metal-chelating agents can be added. Traditionally, chelates were used in agriculture and horticulture to deliver micronutrients to plants. With the use of chelates in phytoremediation, the chelate is used to increase the bioavailability of the contaminant for plant uptake. There are concerns with the use of chelates though. In some situations, the chelate may have a detrimental effect on plant growth. In one experiment, lead hyperaccumulating plants were grown in contaminated soils for two weeks before the chelating agent EDTA was applied. After one week, the plants were harvested after sustaining significant damage (Cunningham et al., 1997). Other experiments utilizing EDTA to increase the bioavailability of lead for phytoextraction have shown a significant increase in the accumulation of lead by even common agronomic plants.
There are other concerns associated with the use of chelates, in addition to the possible detrimental effect on plant health. Much concern has been expressed over the potential of groundwater contamination. The use of chelates will also increase the cost of a remediation process. Some estimates state that to increase the mobility of one ton of lead in contaminated soil will require approximately one ton of EDTA.
Prior to the subject invention, there has been no plant species identified that can accumulate large quantities of arsenic into its biomass. Also, prior to the subject invention there has been no report of the use of ferns in phytoremediation.